What are the functions you find most interesting, useful, and practical?Here are my favourites, mostly stuff I haven't found in other software.

Constant DE threshold, it's awesome on all fronts. No need to change detail level with image size, but mostly I like it because now the fractal is a consistent shape, perspective becomes much more convincing especially with very low detail shapes.

Save image with alpha channel. I am a VJ (video projections at dance parties), having original content that is simple and can be layered up is brilliant.

The Undo button, just because I remember what it was like before it was implemented

Moving lights. Anything that adds surrealism is good.

Glow. By raising the minimum iteration count, transparent shapes can be visualised. My Current focus. Plus it can just look awesome. Hey it would be neat if there was just one glow colour, so that the background of the image isn't washed out. If I have a central shape, the image still has visible borders due to the glow extending to infinity. Also glow gets magnified at limits.

Equirectangular projection. What is really awesome is that this feature was implemented not long after a "dream" in which I had the idea for seamless rolling backgrounds in my visual art.